Diesel engines emit flue gas, which is released to the atmosphere. The flue gas contains pollutants, which include particulate matter (PM) comprising, among others, soot, ash, organic compounds and in many cases sulphur compounds. Sulphur compounds concentration in the flue gas is correlated with the concentration of sulphur compounds in the fuel, typically measured as the sulphur (S) content in the fuel. Fuels differ in their S content from low sulphur ones containing less than 100 ppm S to high sulphur ones containing more than 4% S. The flue gas from burning high S fuels is also high in gaseous sulphur oxides with various S to O ratios, mainly SO2, collectively referred to as SOx. Other undesired gaseous components of the emitted flue gas are nitrogen oxides with various N to O ratios, collectively referred to as NOx.
Large diesel engines operate in both stationary and mobile power generation units. Among the mobile ones, of particular importance are diesel engines operating on board ships and marine oil exploration vessels, also referred to as marine diesel engines. Typically, a ship has in its engine room one or more engines for propulsion purposes (ranging from 4,000 kW to 60,000 kW) and two to four sets of auxiliary engines for electrical power generation or other specific utility purposes. The auxiliary engines typically have a rated power of 500 kW to 1,500 kW). Under normal sea passage, the utilization of the propulsion engines will be between 80% and 85% of MCR (Maximal Continuous Rating) and the required electric power generation will be between 400 kW and 600 kW. Typical marine oil exploration units have several large diesel engines all producing electrical power for propulsion and auxiliary purposes.
Marine diesel engines possess the capability for utilization of high quality fuels (e.g. distillates such as DMA, DMB and DMC according to ISO 8217). Such fuels are quite expensive. Therefore, typically much coarser (lower quality) fuels are utilized. An example of such coarser oil is the heavy fuel oil (HFO), e.g. of ISO 8217 grade characterized by high viscosity, density, carbon, ash and sulphur. The amount of contaminants generated in operating an engine is dependent upon various parameters; such as the type and origin of the fuel, the ambient conditions, the size and speed of the engine, the lubrication system and lubricant consumption, the operating load and the state of maintenance. The term content or amount, when in reference to a contaminating material, may mean its concentration in the effluents, e.g. expressed as weight per weight (w/w), weight per volume (w/v) or volume per volume (v/v). This term may also refer to the amount produced per time of operation (e.g. gram per hour, g/h) or per energy provided (e.g. gram per kilowatt hour, g/kWh). Typically, the flue gas formed when HFO is used in marine diesel engines contains between 1.0 g/kWh and 2.0 g/kWh PM, between 500 ppm and 1,000 ppm SOx and between 8 g/kWh and 17 g/kWh NOx. Higher or lower contents are also found, depending upon the above-listed parameters.
Various methods have been described for minimizing PM, SOx and NOx emissions. Recent patent applications describe SOx removal using a cyclone unit (Israel specification 177901) and PM removal using a cyclone unit (Israel specification 194614).
It is known that recycling of effluent gas, via mixing with the air prior to engine intake, reduces NO formation. Systems using such recycling are implemented in trucks and are referred to as exhaust gas recycle (EGR) systems. Engine modification to incorporate an EGR results in heat absorption by exhaust gas components (CO2) and less O2 density, which contributes to a lower cylinder temperature and reduces NOx formation.
Operating EGR systems for marine diesel engines presents complications related to the impurities in the flue gas formed when burning marine fuels (ISO 8217 grades). PM and SOx present a risk for fouling and corrosion of the turbo charger, of the air cooler and of the scavenging systems employed in marine diesel engines.
Thus, the presence of high levels of PM and SOx negatively impact various engine components, for example, in the turbocharger, where high temperature and corrosion might damage the rotor, rotor shaft and housing.
Similarly, the air cooler is negatively impacted by the presence of high levels of PM and SOx resulting in corrosion.
Reduction of PM and SOx content in the exhaust gas to be recycled is therefore essential and its accomplishment at an acceptable cost presents a major challenge. In addition, while methods for reduction of PM and SOx content are known, they typically fail to totally eliminate those components. The yield of actual elimination is reported in terms of overall percentage. Yet, PM in the exhaust gas differs in size, as well as in chemical composition, chemical properties and physical properties. The yields associated with their removal depend upon such properties and on the method employed for removal. As a result, exhaust gas is enriched in some PM compared with other. PM remaining in the treated gas recycled to EGR systems may be more problematic for the EGR system than the PM removed by existing methods and the same situation may arise with regards to the various types of SOx in the gas. Therefore the prior art does not provide a teaching as to the effect of removal of PM by the prior art methods on the ability of the remaining gas to be used for recycling.
There is therefore a need for improved methods of treating flue gases to enable better removal of impurities, such as PM, SOx and NOx and for the reduction of related costs.